The present invention relates to a system for scanning a target with multiple frequencies of radiation, preferably to enhance the curing of a polymer that is otherwise prevented from receiving direct radiation for photolytic curing due to shadowing.
The shadowing phenomenon occurs when metallization or other opaque structures come between the incident radiation and the polymer to be cured. The inability for the radiation, usually in the form of UV light to reach the polymer, prevents photoinitiators from being activated that normally lead to the polymerization of a photosensitive polymer. This type of problem is especially important in certain types of liquid crystal panel manufacturing where section of glue seal are not accessible to the radiation that affixes the substrates comprising the panel due to certain input/output lines that are deposited in a region directly over the glue seal. In the case of liquid crystal displays, this uncured polymer in the form of a glue seal can lead to contamination of the liquid crystal and a degradation of the panel performance over time.
Alternative methods for curing of sealants are disclosed in U.S. patent appl. Ser. No. 09/425,701 and 09/425,711, both filed on Oct. 22, 1999 in the names of Glownia et al.
One method presently used to cure the polymeric glue that affixes the two substrates that form a liquid crystal display panel is the use of UV radiation to create free radicals that act as photoinitiators in the glue to cause the glue to polymerize or cure. It has been found that any uncured or partially cured glue along the sealant region can cause serious contamination of the liquid crystal leading to long term degradation of the panel""s performance. In the heretofore standard manufacturing process of liquid crystal displays, the two substrates that form the panel are affixed by curing a thermally setting glue at a temperature well above that which the liquid crystal can withstand. This is possible because the liquid crystal is drawn into the panel after the thermal cure step has taken place.
A recently described and preferred panel manufacturing process utilizes the so called xe2x80x98one drop fillxe2x80x99 or xe2x80x98ODFxe2x80x99 method, patented by Matsushita (U.S. Pat. No. 5263888, issued Nov. 23, 1993, entitled, xe2x80x9cMethod of Manufacture of Liquid Crystal Display Panelxe2x80x9d). Here, the liquid crystal is deposited in droplets on one substrate of the two substrates comprising the liquid crystal panel. A narrow fillet of glue seal defines the outer periphery of the liquid crystal material and the second substrate is aligned and placed over the first substrate. At this stage the glue is not polymerized. The conventional oven thermal process to cure or polymerize the glue seal is generally not applicable since the cure temperatures would be detrimental to the liquid crystal now already in place between the two substrates. Instead of thermal curing, a sealant is chosen that polymerizes and cures by way of photoinitiating radicals, rather than by thermal initiators.
The photocuring prevents overheating of the liquid crystal material in contact with the glue seal. To produce the photo-initiated cure reaction, a UV light source is generally used. The problem with this curing process is that the panel contains thin film metal data and gate lines that extend along selected positions from the outer periphery or glue seal region of the panel to the interior of the panel. These signal lines prevent incident UV radiation from reaching the glue seal directly underneath these optically opaque regions leaving the possibility of glue that has not been fully cured. Any glue not fully cured underneath the optically opaque data and gate lines can result in contamination of the liquid crystal material leading to a degradation of panel performance. The contamination is likely caused by the uncured glue that leaches into the liquid crystal region over time.
The present invention simultaneously utilizes several radiation frequencies or wavelengths, separately scanned over portions near the periphery of the panel to overcome the problem of shadowing. In our sealing process different frequencies of the radiation source are used to generate both a photolytic and a thermal reaction, preferably though not necessarily of a dual cure glue seal. The thermal reactions initiated by some portion of the energy available from the radiation source are scanned independently over the regions where shadowing would normally occur. The object is to cure the glue seal with the liquid crystal in place without overheating or degrading the liquid crystal while providing the radiation necessary to react all the glue seal to at least a partial if not a fully polymerized or cured state.
To overcome this problem the aforementioned shadowing problem, our invention in its preferred embodiment uses a high repetition rate pulsed laser with non-linear elements to obtain several frequencies (wavelengths) of radiation. A dual cure glue seal,that is one that cures both thermally and photolytically, is preferably used for the sealant, . UV radiation is commonly used to activate the photolytic reaction while in the present invention, other wavelengths, typically in the visible and the infrared are used to provide local heat in the form of short pulses absorbed by the opaque shadowing patterns. The local heat serves to initiate thermal curing and also lower the viscosity of the glue seal to enhance diffusion of the photoinitiators into the shadowed regions, photoinitiators created by the incident UV light in the regions adjacent to the shadowed areas.
The present invention utilizes two or more monochromatic sources of electromagnetic radiation, with each source of radiation having a different fundamental frequency. These sources or electromagnetic radiation are directed onto a target and scanned along certain paths of the target. The purpose of this scanning may for example be to create photoinitiators which chemically start to cure a polymer that is used to affix two substrates to one another, the substrates forming the aforementioned target.
The sources of radiation, each having different fundamental wavelengths (frequencies) may also be obtained from a single monochromatic source from which different wavelengths can be obtained by using of a non-linear element such as a frequency doubler. The output of the doubling frequency may again be mixed with a second non-linear element to result in a total of three electromagnetic radiation frequencies, that is the fundamental, the fist doubled fundamental, and the a tripled fundamental or fundamental, first and second harmonics. The radiation output at different wavelengths (frequencies) can be spatially separated using, for example, one or more dichroic mirrors or a system of prisms. The separated electromagnetic radiation at the three different wavelengths (frequencies) can then be independently scanned for example by a set of scanning mirrors or optical fibers and the scanned radiation directed onto a target. The target may preferably consist of a pair of substrates that form a liquid crystal display panel. The sources of radiation need not necessarily be monochromatic in which case there is more than one fundamental wavelength (frequency) present for each source. However, for the present application it is preferred to choose the strongest of the multi-line wavelengths ( frequencies) for the wavelength doubling and tripling to obtain the strongest possible output of electromagnetic radiation.
A typical source of monochromatic electromagnetic radiation suitable for the present invention is a laser since both cw and pulsed lasers generally produce electromagnetic radiation with a strong dominant or main fundamental wavelength which can readily be doubled and tripled. For cases where the electromagnetic radiation is to be used to cure a glue seal, ultraviolet wavelengths are often required. Thus, if the tripled wavelength electromagnetic radiation is to be in the wavelength range of 300-400 nm, it is convenient to use a laser whose fundamental wavelength is less than approximately 1.5 microns (1500 nm). Nd3+-YAG or Nd3+-YLF lasers are well suited in that each possess a fundamental electromagnetic radiation wavelength (frequency) that can readily be doubled and tripled. The tripled wavelength (frequency) is in the ultraviolet region and well suited to cure or polymerize the glue seal used to affix the substrates comprising a flat panel display while the fundamental and first doubled wavelength, respectively in the infrared and green-blue region, can be directed to portions on the target that are partially blocked or shadowed by opaque regions on the substrate. While the infrared and blue-green electromagnetic radiation generally cannot polymerize the glue seal, it serves to heat the glue seal beneath the shadowed region. The heating causes a rise in temperature through absorption of the incident radiation by the opaque or partially opaque regions. This temperature rise makes it more likely for the excited radicals formed by the tripled radiation to diffuse underneath the shadowed region to provide a means for curing the shadowed glue seal. These wavelengths (the fundamental and the doubled wavelengths) can be preferably scanned by independent scanning means over relatively short peripheral regions joining the two substrates that are opaque during the time that the ultraviolet or tripled wavelength radiation scans by independently operated scanning means the entire periphery of the two substrates that form the liquid crystal panel. Throughout the discussion the preferred embodiment is that of a liquid crystal, although to those skilled in the art it will be apparent that this is just one example for which the invention is applicable.